Microsoft have had mainly two "families" of Windows development.
There was the simple "DOS" based family that started with Windows 1.0 and went through to Windows 95, then Windows 98 then Windows ME. This family stopped with ME.
Then there was the "server" family that started with Windows NT, then Windows 2000, then became Windows XP.
So XP had a good solid foundation and was based on a fairly secure reliable "core" developped with NT and 2000.
With Vista then Microsoft decided to redesign the whole internals of Windows and throw away much of what they had done in XP and before.
This internal rewrite took MUCH longer than planned, and to be honest was not finished when they shipped the product. It means that the hardware requirement for Vista is probably twice that of XP.
Vista has been such a disaster for Microsoft that there is talk of them trying to rush out the next version of Windows quickly, hoping to bury the public relations disaster that is Vista.